Restore change-detecting trigger infrastructure

[zewelor]

Summary

• restores the removed change-detecting trigger runtime and TriggerState persistence
• brings back dashboard handling, demo data, tests, and AGENTS.md notes for the future trigger work
• intentionally kept as draft so the code is preserved but not merged back into main yet

Verification

• bin/rails test test/jobs/r3x/change_detection_job_test.rb test/lib/r3x/recurring_tasks_config_test.rb test/lib/r3x/dashboard/workflow_summaries_test.rb test/integration/dashboard_test.rb

Context

This PR preserves the infrastructure removed in #73 for later work on the first real change-detecting trigger.